The present invention relates to an autofocusing control system for optical information processing apparatus, and more particularly to an autofocusing control system suitable for use with an optical head for an optical disc device.
In a conventional autofocusing control system for an optical head of an optical disc device, autofocusing control has been made by detecting a displacement of a spot relative to the disc and mechanically moving the focusing lens in accordance with the displacement. Therefore, high precision has been required for the detection performance of a focal displacement detection means and for the response performance of a focusing lens drive means. A more detailed description therefor will be given with reference to FIG. 1 which shows one example of a conventional autofocusing control sytem for an optical head. Light radiated from a semiconductor laser 1 is changed into parallel rays of light at a collimating lens 2. The parallel beam is focused as a spot 4 on a disc 5 by means of a focusing lens 3 having a numerical aperture (NA) of 0.5. Reflected light passes through a .lambda./4 plate 6 and a polarizing beam splitter 7 and is guided to a known focal displacement detection means comprising for example a convex lens 8, a cylindrical lens 9, a knife-edge 10, a 2-divided light detector 11 and a differential circuit 12. The detailed explanation for the detection principle of the detection means is omitted since it is not related to this invention. The focal displacement detection system is disclosed in U.S. Pat. No. 4,450,547. If the disc 5 is positioned at the spot 4, a detection signal 14 of a zero level is outputted. However, if the disc is displaced to a position represented by a reference 5', the reflected light deviates from the parallel beam after passing the lens 3 as shown by broken lines 13. Thus, the differential circuit 12 outputs a detection signal 14 of a certain positive or negative voltage level corresponding to the direction and the amount of the displacement of the disc. In FIG. 2, a solid line 17 shows a relation between the displacement amount of the disc and the output voltage 14 of the differential circuit 12, wherein the abscissa 15 denotes the displacement amount and the ordinate 16 denotes the output voltage 14. Not only the detection means described above, but also most of other focal displacement detection means output a detection signal as shown in FIG. 2. In response to this detection signal 14, the lens 3 is moved in the disc displacement direction by an electromagnetic drive means 18 utilizing for example a voice coil of a loudspeaker.
To stably record or reproduce information on the disc, it is necessary to set the displacement between the spot and the disc within the range of about ##EQU1## assuming that the numerical aperture of the focusing lens NA=0.5 and the wavelength .lambda.=0.83 .mu.m. According to the prior art autofocusing control system, in case the disc displaces about .+-.1 .mu.m, the focal displacement detection means must output a detection signal sufficient for the lens drive means 18 to move the lens 3 at a proper speed and force. Furthermore, while the disc 5 is located within the range of .+-.0.2 to 0.4 .mu.m relative to the spot 4 position, it is necessary to prepare an initial setting adjustment of a target for the focal displacement detection means in such a manner that the detection signal becomes zero level.
In addition, the lens drive means 18 is required to have such high quality response characteristic as the autofocusing control system, as a whole, controls the displacement between the spot 4 and the disc 5 to be limited always within about .+-.1 .mu.m.
As seen from the foregoing, the prior art autofocusing control system has simply employed a method of mechanically moving the focusing lens in order to correct the displacement between the spot 4 and the disc 5. Therefore, such system is associated with some problems that highly precise detection sensitivity and initial setting adjustment for the focal displacement detection means are necessary, as well as a good response characteristic for the lens drive means.